The development of B cells from hematopoietic stem cells to mature functional lymphocytes will be studied using cell surface antigens and physical parameters to deliniate the various stages of differentiation and maturation that these cells go through. The cell populations will be identified and isolated using a Fluorescence Activated Cell Sorter (FACS), and the functional properties of these cells will be tested using an in vitro limiting dilution assay. The developmental stages which can repopulate the immune response will be identified, and the changes in the B cell repertoire resulting from this repopulation will be assessed. A correlation between the various B lymphocyte markers and the ability of a virgin B lymphocyte to react with antigen will be made in order to relate function to expression of cell surface antigen. The kinetics of memory cell generation and the relationship of the memory cell to other cells responding to the same antigen will be made in order to study this phase of antigen dependent B lymphocyte development. The results from these studies will provide information on the entire sequence of development of B lymphocytes from the earliest stage in the bone marrow to the mature primed B cell which is ready to respond the second time an animal encounters the specific antigen. The correlation between immune functions of subpopulations of B lymphocytes and the presence or aquisition of specific surface membrane makers is directly relevant to understanding cell-antigen and cell-cell interactions, and ultimately to development of rational ways for regulating immunity specifically. In the process of these studies general concepts and procedures may be developed which are applicable for studying the differentiation and maturation of other cellular systems.